Terahertz sources produced by ultra-intense laser pulses

Femtosecond laser pulses trigger extreme nonlinear events in matter, leading to intense secondary radiations spanning the frequency ranges from terahertz (THz) to X and gamma-rays.This work is dedicated to the theoretical and numerical study of THz radiation generated by laser-driven plasmas. Despite the inherent difficulty in accessing the THz spectral window (0.1-100 THz), many coming applications use the ability of THz frequencies to probe matter (spectroscopy, medicine, material science). Laser-driven THz sources appear well-suited to provide simultaneously an energetic and broadband signal compared to other conventional devices. Our goal is to investigate previously little explored interaction regimes in order to optimize the laser-to-THz conversion efficiency. Starting from classical interactions in gases, we validate a unidirectional propagation model accounting for THz pulse generation, which we compare to the exact solution of Maxwell's equations. We next increase the laser intensity above the relativistic threshold in order to trigger a nonlinear plasma wave in the laser wake, accelerating electrons to a few hundreds of MeV. We show that the standard photocurrent mechanisms is overtaken by coherent transition radiation induced by wakefield-accelerated electron bunch. Next, successive studies reveal the robustness of this latter process over a wide range of plasma parameters. We also demonstrate the relevance of long laser wavelengths in augmenting THz pulse generation through the ionization-induced pressure that increases the laser ponderomotive force. Finally, THz emission from laser-solid interaction is examined in the context of ultra-thin targets, shedding light on the different processes involved. (author)